Flat-panel display devices employ a variety of technologies for emitting patterned, colored light to form full-color pixels. Some of these technologies employ a common light-emitter for all of the color pixels and color-conversion materials to convert the light of the common light-emitter into colored light of the desired frequencies. Such unpatterned, common light-emitters may be preferred since patterning colored emitters can be difficult. For example, liquid crystal displays (LCDs) typically employ a backlight that relies on either fluorescent tubes to emit a white light or a set of differently colored, inorganic light-emitting diodes to emit white light together with patterned color filters, for example red, green, and blue, to create a full-color display. It is also known to employ the differently colored light-emitting diodes in the set sequentially to create a series of colored backlights in which case color filters may not be necessary. Alternatively, organic light-emitting diodes (OLEDs) may employ a combination of differently colored emitters, or an unpatterned broad-band emitter to emit white light together with patterned color filters, for example red, green, and blue, to create a full-color display. The color filters may be located on the substrate, for a bottom-emitter, or on the cover, for a top-emitter. For example, U.S. Pat. No. 6,392,340 entitled “Color Display Apparatus having Electroluminescence Elements” issued May 21, 2002 illustrates such a device. However, such designs are relatively inefficient since approximately two-thirds of the light emitted may be absorbed by the color filters.
OLEDs rely upon thin-film layers of organic materials coated upon a substrate. OLED devices generally can have two formats known as small-molecule devices such as disclosed in U.S. Pat. No. 4,476,292 and polymer OLED devices such as disclosed in U.S. Pat. No. 5,247,190. Either type of OLED device may include, in sequence, an anode, an organic electroluminescent (EL) element, and a cathode. The organic EL element disposed between the anode and the cathode commonly includes an organic hole-transporting layer (HTL), an emissive layer (EML) and an organic electron-transporting layer (ETL). Holes and electrons recombine and emit light in the EML layer. Tang et al. (Appl. Phys. Lett., 51, 913 (1987), Journal of Applied Physics, 65, 3610 (1989), and U.S. Pat. No. 4,769,292) demonstrated highly efficient OLEDs using such a layer structure. Since then, numerous OLEDs with alternative layer structures, including polymeric materials, have been disclosed and device performance has been improved.
Light is generated in an OLED device when electrons and holes that are injected from the cathode and anode, respectively, flow through the electron transport layer (ETL) and the hole transport layer (HTL) and recombine in the emissive layer (EML). Many factors determine the efficiency of this light generating process. For example, the selection of anode and cathode materials can determine how efficiently the electrons and holes are injected into the device; the selection of ETL and HTL can determine how efficiently the electrons and holes are transported in the device, and the selection of EML can determine how efficiently the electrons and holes be recombined and result in the emission of light, etc.
In yet another alternative means of providing a full-color OLED device, an OLED device may employ a single high-frequency light emitter together with color-conversion materials (also known as color-change materials) layer to provide a variety of color light output. The color-conversion materials absorb the high-frequency light and re-emit light at lower frequencies. For example, an OLED device may emit blue light suitable for a blue sub-pixel and employ a green color-conversion material layer to absorb blue light to emit green light and employ a red color-conversion material layer to absorb blue light to emit red light. The color-conversion materials may be combined with color filters to further improve the color of the emitted light and to absorb incident light and avoid exciting the color-conversion materials with ambient light, thereby improving device contrast. US20050116621 A1 entitled “Electroluminescent devices and methods of making electroluminescent devices including a color-conversion element”, e.g., describes the use of color-conversion materials.
U.S. Patent Application 20040233139A1 discloses a color-conversion member which is improved in the prevention of a deterioration in color-conversion function, the prevention of reflection of external light, and color rendering properties. The color-conversion member comprises a transparent substrate, two or more types of color-conversion layers, and a color-filter layer. The color-conversion layers function to convert incident lights for respective sub-pixels to outgoing lights of colors different from the incident lights. The two or more types of color-conversion layers are arranged on said transparent substrate. The color-filter layer is provided on the transparent substrate side of any one of the color-conversion layers or between the above any one of the color conversion layers and the color conversion layers adjacent to the above any one of the color-conversion layers. US 20050057177 also describes the use of color-conversion materials in combination with color filters.
It has also been found, that one of the key factors that limits the efficiency of OLED devices is the inefficiency in extracting the photons generated by the electron-hole recombination out of the OLED devices. Due to the high optical indices of the organic materials used, most of the photons generated by the recombination process are actually trapped in the devices due to total internal reflection. These trapped photons never leave the OLED devices and make no contribution to the light output from these devices. Because light is emitted in all directions from the internal layers of the OLED, some of the light is emitted directly from the device, and some is emitted into the device and is either reflected back out or is absorbed, and some of the light is emitted laterally and trapped and absorbed by the various layers comprising the device. In general, up to 80% of the light may be lost in this manner.
A typical OLED device uses a glass substrate, a transparent conducting anode such as indium-tin-oxide (ITO), a stack of organic layers, and a reflective cathode layer. Light generated from the device is emitted through the glass substrate. This is commonly referred to as a bottom-emitting device. Alternatively, a device can include a substrate, a reflective anode, a stack of organic layers, and a top transparent cathode layer. Light generated from the device is emitted through the top transparent electrode. This is commonly referred to as a top-emitting device. In these typical devices, the index of the ITO layer, the organic layers, and the glass is about 1.8-2.0, 1.7, and 1.5 respectively. It has been estimated that nearly 60% of the generated light is trapped by internal reflection in the ITO/organic EL element, 20% is trapped in the glass substrate, and only about 20% of the generated light is actually emitted from the device and performs useful functions.
A variety of techniques have been proposed to improve the out-coupling of light from thin-film light emitting devices. For example, diffraction gratings have been proposed to control the attributes of light emission from thin polymer films by inducing Bragg scattering of light that is guided laterally through the emissive layers; see “Modification of polymer light emission by lateral microstructure” by Safonov et al., Synthetic Metals 116, 2001, pp. 145-148, and “Bragg scattering from periodically microstructured light emitting diodes” by Lupton et al., Applied Physics Letters, Vol. 77, No. 21, Nov. 20, 2000, pp. 3340-3342. Brightness enhancement films having diffractive properties and surface and volume diffusers are described in WO0237568 A1 entitled “Brightness and Contrast Enhancement of Direct View Emissive Displays” by Chou et al., published May 10, 2002. The use of micro-cavity techniques is also known; for example, see “Sharply directed emission in organic electroluminescent diodes with an optical-microcavity structure” by Tsutsui et al., Applied Physics Letters 65, No. 15, Oct. 10, 1994, pp. 1868-1870. However, none of these approaches cause all, or nearly all, of the light produced to be emitted from the device. Moreover, such diffractive techniques cause a significant frequency dependence on the angle of emission so that the color of the light emitted from the device changes with the viewer's perspective. Co-pending, commonly assigned U.S. Ser. No. 11/095,166, filed Mar. 31, 2005, describes the use of a micro-cavity OLED device together with a color filter having scattering properties and intended to reduce the angular dependence and color purity of the OLED.
Scattering techniques are also known. Chou (International Publication Number WO 02/37580 A1) and Liu et al. (U.S. Patent Application Publication No. 2001/0026124 A1) taught the use of a volume or surface scattering layer to improve light extraction. The scattering layer is applied next to the organic layers or on the outside surface of the glass substrate and has optical index that matches these layers. Light emitted from the OLED device at higher than critical angle that would have otherwise been trapped can penetrate into the scattering layer and be scattered out of the device. The efficiency of the OLED device is thereby improved but still has deficiencies as explained below.
U.S. Pat. No. 6,787,796 entitled “Organic electroluminescent display device and method of manufacturing the same” by Do et al issued 20040907 describes an organic electroluminescent (EL) display device and a method of manufacturing the same. The organic EL device includes a substrate layer, a first electrode layer formed on the substrate layer, an organic layer formed on the first electrode layer, and a second electrode layer formed on the organic layer, wherein a light loss preventing layer having different refractive index areas is formed between layers of the organic EL device having a large difference in refractive index among the respective layers. U.S. Patent Application Publication No. 2004/0217702 entitled “Light extracting designs for organic light emitting diodes” by Garner et al., similarly discloses use of microstructures to provide internal refractive index variations or internal or surface physical variations that function to perturb the propagation of internal waveguide modes within an OLED. When employed in a top-emitter embodiment, the use of an index-matched polymer adjacent the encapsulating cover is disclosed. US20050142379 A1 entitled “Electroluminescence device, planar light source and display using the same” describes an organic electroluminescence device including an organic layer comprising an emissive layer; a pair of electrodes comprising an anode and a cathode, and sandwiching the organic layer, wherein at least one of the electrodes is transparent; a transparent layer provided adjacent to a light extracting surface of the transparent electrode; and a region substantially disturbing reflection and retraction angle of light provided adjacent to a light extracting surface of the transparent layer or in an interior of the transparent layer, wherein the transparent layer has a refractive index substantially equal to or more than the refractive index of the emissive layer.
Light-scattering layers used externally to an OLED device are described in U.S. Patent Application Publication No. 2005/0018431 entitled “Organic electroluminescent devices having improved light extraction” by Shiang and U.S. Pat. No. 5,955,837 entitled “System with an active layer of a medium having light-scattering properties for flat-panel display devices” by Horikx, et al. These disclosures describe and define properties of scattering layers located on a substrate in detail. Likewise, U.S. Pat. No. 6,777,871 entitled “Organic ElectroLuminescent Devices with Enhanced Light Extraction” by Duggal et al., describes the use of an output coupler comprising a composite layer having specific refractive indices and scattering properties. While useful for extracting light, this approach will only extract light that propagates in the substrate and will not extract light that propagates through the organic layers and electrodes.
It is also known to employ scattering materials within color filters to combine the functions into a single layer. For example, U.S. Pat. No. 6,731,359 describes color filters that include light scattering fine particles and has a haze of 10 to 90. The inclusion of the light-scattering fine particles within the color filter can impart a light scattering function to the color filter per se. This can eliminate the need to provide a front scattering plate on the color filter (in its viewer side). Further, a deterioration in color properties caused by light scattering can be surely compensated for by the color property correction of the colored layer per se and/or by the correction of color properties through the addition of a colorant. This is suitable for surely preventing deterioration in color properties of the color filter per se.
However, scattering techniques, by themselves, cause light to pass through the light-absorbing material layers multiple times where they are absorbed and converted to heat. Moreover, trapped light may propagate a considerable distance horizontally through the cover, substrate, or organic layers before being scattered out of the device, thereby reducing the sharpness of the device in pixellated applications such as displays. A light ray emitted from the light-emitting layer may be scattered multiple times, while traveling through the substrate, organic layer(s), and transparent electrode before it is emitted from the device. When the light ray is finally emitted from the device, the light ray has traveled a considerable distance through the various device layers from the original sub-pixel location where it originated to a remote sub-pixel where it is emitted, thus reducing sharpness. Most of the lateral travel occurs in the substrate, because that is by far the thickest layer in the package. Also, the amount of light emitted is reduced due to absorption of light in the various layers.
U.S. Patent Application Publication No. 2004/0061136 entitled “Organic light emitting device having enhanced light extraction efficiency” by Tyan et al., describes an enhanced light extraction OLED device that includes a light scattering layer. In certain embodiments, a low index isolation layer (having an optical index substantially lower than that of the organic electroluminescent element) is employed adjacent to a reflective layer in combination with the light scattering layer to prevent low angle light from striking the reflective layer, and thereby minimize absorption losses due to multiple reflections from the reflective layer. The particular arrangements, however, may still result in reduced sharpness of the device.
Co-pending, commonly assigned U.S. Ser. No. 11/065,082, filed Feb. 24, 2005, describes the use of a transparent low-index layer having a refractive index lower than the refractive index of the encapsulating cover or substrate through which light is emitted and lower than the organic layers to enhance the sharpness of an OLED device having a scattering element. US 20050194896 describes a nano-structure layer for extracting radiated light from a light-emitting device together with a gap having a refractive index lower than an average refractive index of the emissive layer and nano-structure layer. In various described embodiments, such nano-structure layer may be used in combination with color conversion material or color filter layers. Such disclosed designs still, however, do not completely optimize the use of emitted light, particularly for displays with four-color pixels including a white emitter.
For any practical OLED device, it is important to minimize the cost and maximize the yield of manufacturing the device. Applicants have determined through experience and experiment, that the integration of the various components can be complex and the various layers may have process incompatibilities. There is a need therefore for improved organic light-emitting diode devices, and processes for forming such devices that provide reduced cost and improved yields.